When you want to repeat a famous proverb or saying, introduce it by saying "You know what they say:" For example:
You know what they say: if it's not broke, don't fix it!
You know what they say: absence makes the heart grow fonder.
It's common just to say the first part of the famous saying. Everyone knows the rest of it, so it's not necessary to say it all. For example:
You know what they say: When in Rome...
The rest of the saying is "When in Rome, do as the Romans."